A study of the influence of dietary lipid composition on development of spontaneous mammary tumors in C3H mice is proposed. Ten rations containing single fats or oils and 10 additional diets containing combinations of various fats and oils will be fed. The latter diets will be formulated such that the lowest possible correlaton ratios between fatty acids in each diet will be achieved. In this way the development of tumors can be correlated to the dietary fat type as well as specific dietary fatty acids by means of regression analysis. An additional 4 groups of mice will receive diets containing various fats as well as selenium to determine possible interactions between dietary fats and selenium as related to tumor development. Lipid composition of tumors, nonneoplastic mammary tissue and liver tissue from mice fed each diet will be determined. Selenium and glutathione peroxidase determinations will be made on mice from appropriate groups.